Surface acoustic wave filter

Abstract

A surface acoustic wave filter unit having three interdigital transducers arranged along the surface acoustic wave propagation direction is disposed on a piezoelectric substrate. An unbalanced signal terminal and balanced signal terminals are provided for the surface acoustic wave filter unit. At least one of the three interdigital transducers is out of phase relative to the other interdigital transducers. Reflectors are arranged so as to sandwich the three interdigital transducers therebetween. The reflectors are grounded. Thus, a surface acoustic wave filter having a balance-to-unbalance conversion function and having high balance between the balanced signal terminals is achieved.

What is claimed is:  
     
       1. A surface acoustic wave filter comprising: 
       a piezoelectric substrate;  
       a longitudinally coupled resonator filter provided on the piezoelectric substrate, said longitudinally coupled resonator filter including at least three interdigital transducers arranged along a surface acoustic wave propagation direction, at least one of the at least three interdigital transducers has a different phase from the other interdigital transducers;  
       an unbalanced terminal and a balanced terminal which are provided for the longitudinally coupled resonator filter; and  
       reflectors arranged to sandwich the at least three interdigital transducers therebetween in the longitudinally coupled resonator filter, each of said reflectors being grounded.  
     
     
       2. A surface acoustic wave filter according to  claim 1 , wherein the piezoelectric substrate is one of a 40±5° Y-cut X-propagating LiTaO 3  substrate, a 64° to 72° Y-cut X-propagating LINbO 3  substrate, and a 41° Y-cut X-propagating LiNbO 3  substrate. 
     
     
       3. A surface acoustic wave filter according to  claim 1 , wherein at least one surface acoustic wave resonator is connected at least in series or in parallel to the longitudinally coupled resonator filter. 
     
     
       4. A surface acoustic wave filter according to  claim 1 , wherein each of the reflectors has 20 to 200 electrode fingers. 
     
     
       5. A surface acoustic wave filter according to  claim 1 , wherein the pitch between the interdigital transducers and the reflectors is within a range from about 0.4 AR to about 0.6 AR, where AR represents the wavelength of the reflectors. 
     
     
       6. A surface acoustic wave filter according to  claim 1 , wherein the interdigital transducers located at both sides of the center interdigital transducer among the at least three interdigital transducers in the longitudinally coupled resonator filter have an inverted phase relative to each other. 
     
     
       7. A surface acoustic wave filter according to  claim 1 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at feast three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers. 
     
     
       8. A surface acoustic wave filter comprising: 
       a piezoelectric substrate;  
       a plurality of longitudinally coupled resonator filters provided on the piezoelectric substrate, each filter including at least three interdigital transducers arranged along a surface acoustic wave propagation direction, at least one of the at least three interdigital transducers has a different phase from the other interdigital transducers;  
       an unbalanced terminal and a balanced terminal which are provided for the plurality of longitudinally coupled resonator filters; and  
       reflectors arranged to sandwich the at least three interdigital transducers therebetween in each of the longitudinally coupled resonator filters, each of said reflectors being grounded.  
     
     
       9. A surface acoustic wave filter according to  claim 8 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at least three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers. 
     
     
       10. A surface acoustic wave filter according to  claim 8 , wherein the piezoelectric substrate is one of a 40±5° Y-cut X-propagating LiTaO 3  substrate, a 64° to 72° Y-cut X-propagating LiNbO 3  substrate, and a 41° Y-cut X-propagating LiNbO 3  substrate. 
     
     
       11. A surface acoustic wave fitter according to  claim 8 , wherein at least one surface acoustic wave resonator is connected at least in series or in parallel to at least one of the longitudinally coupled resonator filters. 
     
     
       12. A surface acoustic wave filter according to  claim 8 , wherein each of the reflectors has 20 to 200 electrode fingers. 
     
     
       13. A surface acoustic wave filter according to  claim 8 , wherein the pitch between the interdigital transducers and the reflectors is within a range from about 0.4 AR to about 0.6 AR, where AR represents the wavelength of the reflectors. 
     
     
       14. A surface acoustic wave filter according to  claim 8 , wherein the interdigital transducers located at both sides of the center interdigital transducer among the at least three interdigital transducers in the longitudinally coupled resonator filter have an inverted phase relative to each other. 
     
     
       15. A surface acoustic wave filter according to  claim 10 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at least three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers. 
     
     
       16. A surface acoustic wave filter according to  claim 8 , wherein the plurality of longitudinally coupled resonator filters are cascade-connected. 
     
     
       17. A surface acoustic wave filter according to  claim 11 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at least three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers. 
     
     
       18. A surface acoustic wave filter according to  claim 12 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at least three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers. 
     
     
       19. A surface acoustic wave filter according to  claim 13 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at least three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers. 
     
     
       20. A surface acoustic wave filter according to  claim 14 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at least three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers. 
     
     
       21. A surface acoustic wave filter according to  claim 16 , wherein the at least three interdigital transducers include electrode fingers which are arranged such that a pitch between a first group of the electrode fingers at a portion where the at least three interdigital electrodes face each other is narrower than a pitch between the remaining electrode fingers of the at least three interdigital transducers.